The invention relates to a process for the treatment of resinous polymers of the vinyl aromatic/conjugated diene block copolymer type. More particularly the invention relates to a process for the treatment of such copolymers obtained starting from a (co)polymerization--initiating stage performed by means of alkaline metal based catalysts (Me).
French Patent 1,149,159 discloses a process for removing the deep color, due to hydrogenation catalysts, of hydrogenated rubber polymers by means of a mineral acid or of a monocarboxylic or polycarboxylic acid; the result is transparent but yellowish polymers. Further it was found that the decolored polymers according to this process tend to absorb water which causes an increase of their opacity with time.
European Patent 479,754 discloses a process for the decoloration of resinous polymers of the vinyl aromatic/conjugated diene type consisting in the treatment of these copolymers with a monocarboxylic acid having from 3 to 5 carbon atoms.
However, in the case of copolymers of resinous types, more particularly those disclosed in U.S. Pat. Nos. 3,639,517 and 4,091,053, the coloring is mainly due to the initiating agents. Not only for the appearance, but also for uses in the field of foodstuffs, it is very important to make this yellowish coloring disappear and to obtain completely transparent copolymers from which the color has been removed.
Methods are known for obtaining transparent copolymers by treating solutions of the copolymers with CO.sub.2 and water. However, this requires a distillation of the solvent to separate it from the water and the CO.sub.2 before it is recycled for polymerization, since otherwise these products would act as poisons to the initiator.
Further, it must be remembered that as well as transparency and decoloration, the polymer must maintain all its mechanical shock resisting properties, something which is not ensured by the aforedescribed processes.
European Patent 84,795 also suggests the treatment of copolymers with carborylic diacids. That type of process enables practically all the aforementioned decoloration and mechanical properties stability conditions to be met, but without solving the above-identified problem of the water absorption.
It would therefore seem desirable to obtain a process for the treatment of vinyl aromatic/conjugated diene copolymers in such a way to ensure that they remain completely transparent, retain appropriate shock-resistant properties, can be brought into contact with foodstuffs and have an improved resistance to the water absorption.